During the combustion of fossil fuels, sulphur in the fuel is released in form of sulphur oxides (SOX). Other pollutants are primary particulate matter and nitrogen oxides (NOX). It is well known that air pollution seriously impact people's health and the environment. It is also well known that sulphur dioxide and nitrogen oxides are the major precursors of acid rain.
So far, the legislation that has been setting the environmental standards for emission control for international shipping has lagged behind the land-based emission regulations. Present regulations include caps on the sulphur content of fuel oil as a measure to control SOX emissions. Special fuel-quality provisions exist for SOX in emission control areas and there are substantial reductions in the allowed fuel sulphur limits to be expected in the near future. The MARPOL Annex VI legislation, which went into effect in May 2005 following specifications from several European Union directives, has curbed the impact of marine diesel on the environment. By 2015, the legislations will be even stricter regarding by way of example fuel sulphur limits and NOX cap.
There are different emission reduction possibilities, either taken alone or in combination. One possibility is to use new fuels such as distillate fuels or low sulphur fuels. Another possibility is to further develop methods to control the emission of SOX such as wet scrubber technologies using alkaline agents such as NaOH-solutions, or dry scrubber technologies using granulated limestone (Ca(OH)2).
It is well known in the marine industry today for the purpose of reducing SOX gases in the exhaust from a ship to apply Exhaust Gas Cleaning (EGC) for SOX reduction and Exhaust Gas Recirculation (EGR) for SOX and NOX reduction. Both the EGC and EGR processes may apply a wet scrubber technology using an alkaline agent such as a NaOH-solution for removal of SOX. However, since the EGC process is applied on the low pressure side of the exhaust source and the EGR process is applied on the high pressure side it is not possible for them to share the same wet scrubber.
In the closed loop of a scrubber system, no matter if it is a scrubber to be operated as an EGC scrubber or as an EGR scrubber, the water is circulated through the scrubber absorbing SO2 and the water is afterwards balanced with an alkaline agent such as NaOH to an almost neutral pH in the range of 7-8 before being circulated back to the scrubber. It is very important that the pH does not rise too high above pH 8, in that the scrubber will then start absorbing CO2 from the exhaust. If this happens, the consumption of the alkaline agent will increase and hence the operational costs.
Another issue in the closed loop of a scrubber system is the so called Chemical Oxygen Demand (COD) which is a value describing how much oxygen is required to oxidize all organic and inorganic matter present in the scrubber water. A high COD value will in worst case lead to decreased oxygen levels in the oceans.
U.S. Pat. No. 4,085,194 discloses a method of removing sulphurous acid gas from smokes discharged from a variety of combustion facilities such as generating power plants. The method includes the step of oxidizing the sulphites into chemically stable sulphates by introducing air into the sea-water. The air is introduced by a rotating umbrella-type rotor which is submerged in a body of the sulphite-containing sea-water.
When SO2 is absorbed in the water it undergoes the following three reactions; reaction 1 describes the absorption of SO2 into the water phase, reaction 2 is the neutralization with NaOH and reaction 3 describes the oxidation of the absorbed SO2 to its final inactive stage.SO2(g)+H2O(l)→H2SO3(aq)  (1)H2SO3(aq)+2*NaOH(aq)→Na2SO3(aq)+2*H2O(l)  (2)Na2SO3(aq)+½*O2(aq)→Na2SO4(aq)  (3)
Reactions 1 and 2 are fast, while reaction 3 is slow with an estimated half-life of at least 10 minutes provided enough oxygen is present. If all dissolved oxygen is consumed, reaction 3 will stop completely. When the final stage with Na2SO4 is reached, the sulphur is stabilized and the absorption process is finished.
After several hours of operation in the closed loop mode, the contents of Na2SO4 in the scrubber water has reached a level under which precipitation of Na2SO4 is a real risk. A discharge of the scrubber water from the scrubber system is thereby required.
Further, during the absorption process, oil and soot residues from the combustion process are transferred from the exhaust gas to the scrubber water. This increases both turbidity and COD values.
According to current legislations regarding discharge of scrubber water, several discharge criteria must be fulfilled, three of these discharge criteria's are of specific interest for this invention:                The pH value should be larger than 6.5.        The turbidity (solid contents) should be below 25 NTU/FNU.        A low COD value. No specific limit has yet been determined but it is expected to be similar to IMO/Marpo Annex 4, i.e. 125 mg/L.        